Recently, many transparent touch panels capable of enabling an interactive input system have been put into use. The transparent touch panel includes, for example, an optical system, an ultrasonic system, a capacitance system and a resistance film system, according to the position-sensing system. Of these, the resistance film system has a simple structure and excellent price/performance ratio, and therefore has gained widespread use in recent years.
The touch panel of a resistance film system is an electronic component fabricated by holding two films or sheets having respective transparent electroconductive layers on the opposing sides with a constant distance therebetween, wherein a movable electrode substrate (electrode substrate on the viewing side) is pressed with a pen or a finger to sag and cause contact and electrical conduction with a fixed electrode substrate (electrode substrate on the opposite side), and a sensor circuit is then allowed to detect the position, thereby effecting a predetermined input. At this time, an interference fringe called a Newton ring sometimes appears around the pressed part. Also, even in a non-pressed state, a Newton ring may appear in a portion where the distance between the movable electrode substrate and the fixed electrode substrate is decreased due to sagging of the movable electrode substrate. The generation of a Newton ring impairs the visibility of a display.
In order to reduce the Newton ring that may be generated between two transparent electrode substrates constituting such a transparent touch panel of a resistance film system, a method of forming an unevenness with an appropriate shape and an appropriate size on the film surface is effective. Specifically, there are disclosed a method where a coating layer containing a predetermined amount of a filler having an average primary particle diameter of 1 to 4 μm and a transparent electroconductive layer formed on a plastic film (see, Japanese Unexamined Patent Publication No. 10-323931), and a method where a projected coating layer (coating layer having projections) containing silica particles having an average secondary particle diameter of 1.0 to 3.0 μm is formed on a plastic film (see, Japanese Unexamined Patent Publication No. 2002-373056).
In the case of a touch panel using a transparent electroconductive laminate where, as described above, a coating layer containing particles having an average primary particle diameter or average secondary particle diameter of about several microns and a transparent electroconductive layer are formed on a plastic film, the generation of a Newton ring is reduced. However, when this transparent touch panel is installed on a high-definition display, the resin around a particle in the coating layer above exerts a lens effect to cause a problem that color separation (sparkling) of light coming from the display occurs and then the visibility of the display is seriously impaired.
As other coating layers for reducing the Newton ring, there is disclosed a method where inorganic fine particles having an average primary particle diameter of 100 nm or less are added to a cured resin containing inorganic fine particles having an average primary particle diameter of 0.5 to 5 μm, whereby the unevenness profile is controlled, and the generation of a Newton ring and the impairment of visibility due to sparkling are reduced simultaneously (see, Japanese Unexamined Patent Publication No. 2006-190512). However, the anti-Newton ring layer formed by this method has a problem that when a test of sliding durability or edge-pressing durability required of a touch panel is performed, the transparent electroconductive layer starts deteriorating and separating from the projection portion formed by the inorganic fine particle, and the electrical characteristics as a touch panel are degraded at the end.
Also, when a hitting durability test is performed, the projection formed by the inorganic fine particle contained in the transparent electroconductive layer-forming surface of the movable electrode substrate destroys a dot spacer formed on the fixed electrode substrate to scatter broken pieces in the touch panel. The thus-scattered broken pieces of the dot spacer disadvantageously prevent the electric connection between the movable electrode substrate and the fixed electrode substrate, and deteriorate the electrical characteristics of the touch panel. Furthermore, there is a problem that the scattered broken pieces of the dot spacer adhere to and damage the transparent electroconductive layers of the movable electrode substrate, and the fixed electrode substrate and the electrical characteristics of the touch panel are degraded.
In addition, when such an anti-Newton ring layer formed using inorganic fine particles is used as a fixed electrode substrate, there is also a problem that the projection formed by the inorganic fine particle damages the transparent electroconductive layer of the movable electrode substrate, and the electrical characteristics of the touch panel are degraded. Also, when a resin layer containing fine particles of about several μm is formed by wet coating using a gravure coater or the like, fine particles in the coating solution precipitate with aging, and this requires frequent change of the coating solution, which results in a problem in the productivity.
As the method of not using an inorganic fine particle of 1 μm or more to form an unevenness profile on the film surface, for example, a thermoplastic resin and an inorganic fine particle having an average primary particle diameter of 0.001 μm to less than 1 μm are combined with a compound polymerizable upon irradiation with an activation energy ray or an oligomer thereof, but in a layer formed by such a technique, the haze becomes extremely high, and thereby the visibility of a display is impaired (see, Japanese Unexamined Patent Publication No. 2002-275391).
Also, a method where ultrafine particles having an average primary particle diameter of 100 nm or less are dispersed as an aggregate of less than 1.0 μm or in a state of not forming an aggregate in a cured resin layer to form an unevenness profile is disclosed, but the unevenness of an anti-Newton ring layer formed by such a technique is small, and when the pressing pressure is strong, a Newton ring is observed (see, Japanese Unexamined Patent Publication Nos. 2006-056136, 2005-209431, 2004-351744 and 2004-195898).